Etching composition, method for etching insulating film of semiconductor devices using the same and method for preparing semiconductor devices

ABSTRACT

An etching composition includes phosphoric acid, a silane compound comprising at least one silicon (Si) atom, and an ammonium salt represented by Formula 1 below: 
     
       
         
         
             
             
         
       
         
         
           
             wherein: 
             L 1  to L 3  are independently substituted or unsubstituted hydrocarbylene, 
             R 1  to R 4  are independently hydrogen, a substituted or unsubstituted hydrocarbyl group, and 
             X n−  is an n-valent anion, where n is an integer of 1 to 3.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims benefit of priority to Korean Patent Application No. 10-2019-0109544 filed on Sep. 4, 2019, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND 1. Field

The present disclosure relates to an etching composition, particularly, to an etching composition having a high selectivity ratio, capable of selectively removing a nitride film while minimizing an etching rate of an oxide film.

2. Description of Related Art

An oxide film, such as a silicon oxide (SiO₂) film, and a nitride film, such as a silicon nitride (SiNx) film, are representative insulating films. In a semiconductor manufacturing process, the silicon oxide film and the silicon nitride film are used alone or in a form in which one or more films are alternately stacked. In addition, the oxide film or the nitride film is also used as a hard mask for forming a conductive pattern such as a metal wiring.

In a wet etching process for removing the nitride film, a mixture of phosphoric acid and deionized water is generally used. The deionized water is added for preventing a decrease in an etching rate and a change in etching selectivity to an oxide film; however, there may be a problem arising in that defects may occur in a nitride film etching removal process, even with a minute change in an amount of supplied deionized water. In addition, the phosphoric acid is a strong acid and corrosive, thereby leading to difficulties in handling.

In order to solve the above problems, there is a conventionally known technology for removing a nitride film using an etching composition containing fluoric acid (HF), nitric acid (HNO₃), or the like, in phosphoric acid (H₃PO₄). This technology, however, results in reducing an etching selectivity ratio of the nitride film to the oxide film.

Further, there is also a known technology of using an etching composition including phosphoric acid and a silicic acid or a silicate. However, the silicic acid or silicate has a problem of generating particles which may affect a substrate, thereby being inappropriate for a semiconductor manufacturing process.

In the meantime, when phosphoric acid is used in a wet etching process for removing the nitride film, not only the nitride film but also an oxide film such as an SOD oxide film may be etched due to a reduced etching selectivity ratio of the nitride film to the oxide film, whereby it may be difficult to adjust an effective field oxide height (EFH). Accordingly, a sufficient wet etching time for removing the nitride film may not be secured, or an additional process may be required, causing a change and having adverse effects thereon.

Therefore, an etching composition having a high selectivity ratio, which selectively etches a nitride film to an oxide film and does not have a problem such as particle generation in a semiconductor manufacturing process, is currently demanded.

Meanwhile, a silane-based additive, which is an additive added to a conventional etching composition, has solubility too low to secure optimal solubility, thereby causing precipitation of particles in the etching solution composition and abnormal growth of a substrate. Such particles may remain on a silicon substrate, resulting in a defect of a device implemented thereon, or in failure of equipment used in an etching or a washing process.

Further, when the etching composition is stored for a long period time, etching speeds of a nitride film and a silicon oxide film changes, thereby varying selection ratio of the nitride film to the oxide film.

SUMMARY

An aspect of the present disclosure is to provide an etching composition having a high selectivity ratio, which can selectively remove a nitride film while minimizing etching of an oxide film, without causing problems such as particle generation having adverse effects on device characteristics.

Another aspect of the present disclosure is to provide an etching composition having excellent storage stability.

Further, another aspect of the present disclosure is to provide a method for etching an insulating film using the etching composition and a method for manufacturing a semiconductor device.

The present disclosure is to provide an etching composition. According to an embodiment of the present disclosure, an etching composition includes phosphoric acid, a silane compound containing at least one silicon (Si) atom, and an ammonium salt represented by Formula 1:

In Formula 1, L¹ to L³ are independently substituted or unsubstituted hydrocarbylene, R¹ to R⁴ are independently hydrogen, a substituted or unsubstituted hydrocarbyl group, and X^(n−) is an n-valent anion, where n is an integer of 1 to 3.

The X may be one selected from the group consisting of a halogen, C₁-C₁₀ carboxylate, tosylate, sulfate, mesylate, bisulfate, carbonate, bicarbonate, phosphate, hydrogen phosphate, dihydrogen phosphate and nitrate.

L¹ to L³ may be independently substituted or unsubstituted C₁-C₅ alkylene, and are preferably the same. For example, L¹ to L³ may be —CH₂CH₂— or —CH₂CH(CH₃)—.

The hydrocarbyl group may be a substituted or unsubstituted C₁-C₂₀ alkyl group or a substituted or unsubstituted C₅-C₂₀ aryl group.

For example, R¹ to R³ may be hydrogen, and R⁴ may be hydrogen or a C₁-C₂₀ alkyl group. More specifically, R⁴ may be hydrogen or a —CH₃.

The ammonium salt of Formula 1 may be selected from following Structural Formulae 1 to 14:

where OAc, OTs and OMs represent acetate, tosylate and mesylate, respectively.

The silane compound may be represented by Formula 2 below:

In Formula 2, R⁵¹ to R⁵⁴ are independently hydrogen, a substituted or unsubstituted hydrocarbyl group or a substituted or unsubstituted heterohydrocarbyl group, and R⁵¹ to R⁵⁴ exist independently or two or more thereof form a ring via a heteroatom.

The etching composition of each example embodiment above may include 70 wt % to 95 wt % of the phosphoric acid, 0.001 wt % to 5 wt % of the silane compound and 0.001 wt % to 10 wt % of the ammonium salt represented by Formula 1.

The present disclosure provides a method for etching an insulating film using the etching composition described above, and a method for preparing a semiconductor device including the method for etching an insulating film.

BRIEF DESCRIPTION OF DRAWINGS

The above and other aspects, features and other advantages of the present disclosure will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIGS. 1 and 2 are exemplary process cross-sectional views illustrating a device separation process of a flash memory device.

DETAILED DESCRIPTION

The present disclosure is to provide an etching composition, in particular, an etching composition having a high selectivity ratio enabling selective removal of a nitride film while minimizing etching of an oxide film and having excellent storage stability.

The etching composition of the present disclosure includes phosphoric acid. The phosphoric acid may react with silicon nitride to etch a silicon nitride film, and react with the silicon nitride as in Equation (1) below:

3Si₃N₄+27H₂O+4H₃PO₄→4(NH₄)₃PO₄+9SiO₂H₂O  (1).

The etching composition of the present disclosure includes an ammonium salt represented by Formula 1 below:

In Formula 1, L¹ to L³ are independently substituted or unsubstituted hydrocarbylene. L¹ to L³ may be independently substituted or unsubstituted C₁-C₅ alkylene, for example, —CH₂CH₂— or —CH₂CH(CH₃)—. L¹ to L³ may be different from each other or the same.

In Formula 1, R¹ to R⁴ are independently hydrogen, a substituted or unsubstituted hydrocarbyl group. The substituted or unsubstituted hydrocarbyl group may be a substituted or unsubstituted C₁-C₂₀ alkyl group or a substituted or unsubstituted C₅-C₂₀ aryl group. R¹ to R⁴ may be different from each other, or two or more thereof may be the same. For example, R¹ to R³ may be hydrogen, and R⁴ may be hydrogen or a C₁-C₂₀ alkyl group.

In Formula 1, X^(n−) may be an n-valent anion, and preferably where n is an integer of 1 to 3. For example, the X may be a halogen, such as F, Cl, Br, I, or the like. X may also be one selected from the group consisting of C₁-C₁₀ carboxylate such as an acetate, tosylate, sulfate, mesylate, bisulfate, carbonate, bicarbonate, phosphate, hydrogen phosphate, dihydrogen phosphate and nitrate.

The ammonium salt represented by Formula 1 provided in the present disclosure provides structural stability to the silane compound and prevents decomposition or abnormal growth of the silane compound to suppress particle formation of the etching compound and etching of the silicon oxide.

The ammonium salt contained in the etching composition may be used alone as long as it is represented by Formula 1 above, or two or more thereof may be mixed to use.

The ammonium salt employed in the present disclosure may be an ammonium salt of the following structures (1) to (14):

where OAc, OTs and OMs represent acetate, tosylate and mesylate, respectively.

The ammonium salt of Formula 1 may prevent particle formation of the etching composition and may be added in an amount of 0.001 wt % to 10 wt % based on a total weight of the etching composition. When the ammonium salt is added in an amount of less than 0.001 wt %, it is difficult to obtain an effect of suppressing particle formation, whereas if the amount exceeds 10 wt %, the ammonium salt may accelerate particle formation.

A content of the ammonium salt of Formula 1 may be, for example, 0.001 to 5 wt %, 0.001 to 3 wt %, 0.001 to 2 wt %, 0.001 to 1 wt %, 0.005 to 5 wt %, 0.005 to 3 wt %, 0.005 to 2 wt %, 0.005 to 1 wt %, 0.01 to 5 wt %, 0.01 to 3 wt %, 0.01 to 2 wt %, 0.01 to 1 wt %, 0.05 to 5 wt %, 0.05 to 3 wt %, 0.05 to 2 wt %, 0.05 to 1 wt %, 0.1 to 5 wt %, 0.1 to 3 wt %, 0.1 to 2 wt %, 0.1 to 1 wt %, 0.5 to 5 wt %, 0.5 to 3 wt %, 0.5 to 2 wt % or 0.5 to 1 wt %.

The etching composition includes a silane compound. Any silane compound containing one or more silicon atoms may be appropriately used in the present disclosure. More preferably, the silane compound may be a silane compound represented by Formula 2 below:

In Formula 2, R⁵¹ to R⁵⁴ are independently hydrogen, a substituted or unsubstituted hydrocarbyl group, such as a substituted or unsubstituted C₁-C₂₀ hydrocarbyl group, or a substituted or unsubstituted heterohydrocarbyl group, such as a substituted or unsubstituted C₁-C₂₀ heterohydrocarbyl group. R⁵¹ to R⁵⁴ may exist independently, or two or more thereof may form a ring connected by a heteroatom. For example, R⁵¹ to R⁵⁴ may be hydrogen, a substituted or unsubstituted C₁-C₂₀ alkyl group, a substituted or unsubstituted C₁-C₂₀ heteroalkyl group, or the like. In this case, the heteroatom is not particularly limited, but may be N, S, O, P, or the like.

The silane compound represented by Formula 2 may be included in an amount of 0.005 wt % to 1 wt %, based on a total weight of the etching composition.

Further, the etching composition may further include an additional ammonium salt, in addition to the ammonium salt represented by Formula 1.

The ammonium salt is a compound having an ammonium ion, and any of those conventionally used in the art may be appropriately used in the present disclosure. Although not particularly limited, the ammonium salt may be, for example, ammonia water, ammonium chloride, ammonium acetate, ammonium phosphate, ammonium peroxydisulfate, ammonium sulfate, ammonium fluorate, or the like. These may be used alone or in a combination of two or more thereof.

Furthermore, the etching composition of the present disclosure may further contain an optional additive conventionally used in the art to further improve etching performance. The additive may be a surfactant, a metal ion sequestrant, a corrosion inhibitor, or the like.

The etching composition of the present disclosure is used for selective removal of a nitride film by etching from a semiconductor device including an oxide film and the nitride film. The nitride film may include a silicon nitride film, for example, a SiN film, a SiON film, or the like.

In addition, the oxide film may be at least one selected from the group consisting of a silicon oxide film, for example, a spin on dielectric (SOD) film, a high density plasma (HDP) film, a thermal oxide film, a borophosphate silicate glass (BPSG) film, a phosphosilicate glass (PSG) film, a borosilicate glass (BSG) film, a polysilazane (PSZ) film, a fluorinated silicate glass (FSG) film, a low pressure tetraethyl orthosilicate (LPTEOS) film, a plasma enhanced tetraethyl orthosilicate (PETEOS) film, a high temperature oxide (HTO) film, a medium temperature oxide (MTO) film, an undoped silicate glass (USG) film, a spin on glass (SOG) film, an advanced planarization layer (APL) film, an atomic layer deposition (ALD) film, a plasma enhanced oxide (PE-oxide) film, an O3-tetraethyl orthosilicate (O3-TEOS) film or combinations thereof.

An etching method involving use of the etching composition of the present disclosure may be performed by a wet etching method, for example, dipping, spraying, or the like.

An example of an etching process using the etching composition of the present disclosure is schematized in FIGS. 1 and 2. FIGS. 1 and 2 are exemplary process cross-sectional views illustrating a device separation process of a flash memory device.

As illustrated in FIG. 1, a tunnel oxide film 11, a polysilicon film 12, a buffer oxide film 13 and a pad nitride film 14 are formed on a substrate 10 in said order, and the polysilicon film 12, the buffer oxide film 13 and the pad nitride film 14 are then selectively etched to form a trench. Subsequently, an SOD oxide film 15 is formed until the trench is gap-filled, and a CMP process is then carried out on the SOD oxide film 15 using the pad nitride film 14 as a polishing stop film.

As illustrated in FIG. 2, after the pad nitride film 14 is removed by wet etching using a phosphoric acid solution, the buffer oxide film 13 is removed in a washing process. As a result, a device separation film 15A is formed in a field area.

A temperature of the etching process may be in a range of 50° C. to 300° C., preferably 100° C. to 200° C., more preferably 156° C. to 163° C., and an appropriate temperature may be changed, if necessary, in consideration of other processes and factors.

As such, according to a method for manufacturing a semiconductor device including the etching process carried out using the etching composition of the present disclosure, selective etching of the nitride film is feasible when the nitride film and the oxide film are alternately stacked or mixed. In addition, stability and reliability of the process can be secured by preventing the generation of particles, which was problematic in the conventional etching process.

Accordingly, such a method may be efficiently applied to various processes requiring selective etching of the nitride film to the oxide film in the semiconductor device manufacturing process.

EXAMPLE

Hereinafter, the present disclosure will be described in detail by way of examples. The following Examples relate to an example of the present disclosure, but the present disclosure is not limited thereto.

Synthesis Example 1

10 g of triethanolamine was added to a 100 mL round bottom flask, followed by adding 30 mL of ethanol, and stirred.

The mixture was cooled until reaching 0° C. while maintaining an N₂ atmosphere, and 4.1 g of acetic acid was gradually added. A temperature outside of the flask was increased to 90° C., and the mixture was reacted under reflux for 24 hours.

The external temperature was lowered to 40° C., and the ethanol was removed by distillation under reduced pressure. Vacuum drying was then performed for 12 hours to obtain an ammonium salt additive 1 of Formula 1 below:

¹H-NMR (DMSO-d6): 5.00 (broad, 4H), 3.71 (t, 6H), 2.67 (t, 6H), 1.98 (s, 3H)

Synthesis Example 2

10 g of triethanolamine was added to a 100 mL round bottom flask, followed by adding 30 mL of acetonitrile, and stirred.

The mixture was cooled until reaching 0° C. while maintaining an N₂ atmosphere, and 4.3 g of dimethylsulfate was gradually added. A temperature outside of the flask was increased to 90° C., and the mixture was reacted under reflux for 60 hours.

The external temperature was lowered to 25° C., and a layer of the acetonitrile was removed, followed by adding another 30 mL of acetonitrile to the flask to wash-remove the same.

Vacuum drying was then performed for 12 hours to obtain an ammonium salt additive 2 of Formula 2 below:

¹H-NMR (DMSO-d6): 4.78 (broad, 3H), 3.81 (m, 6H), 3.49 (t, 6H), 3.12 (s, 3H)

Synthesis Example 3

5 g of triisopropanolamine was added to a 100 mL round bottom flask, followed by adding 30 mL of ethanol, and stirred.

The mixture was cooled until reaching 0° C., and 1.65 mL of 70% nitric acid was gradually added. A temperature outside of the flask was increased to 90° C., and the mixture was reacted under reflux for 20 hours.

The external temperature was lowered to 50° C., and the ethanol was removed by distillation under reduced pressure. 50 mL of toluene was added to a resulting concentrated residue and distilled again under reduced pressure. Vacuum drying was then performed for 24 hours to obtain an ammonium salt additive 3 of Formula 3 below:

¹H-NMR (DMSO-d6): 4.94 (broad, 4H), 3.78 (m, 3H), 2.61 (m, 6H), 1.31 (m, 9H)

Synthesis Example 4

10 g of triisopropanolamine was added to a 100 mL round bottom flask, followed by adding 30 mL of ethanol, and stirred.

The mixture was cooled until reaching 0° C., and 9.7 g of methyl p-toluenesulfonate was gradually added. A temperature outside of the flask was increased to 90° C., and the mixture was reacted under reflux for 24 hours.

The external temperature was lowered to 50° C., and the ethanol was removed by distillation under reduced pressure. 50 mL of acetone was added to a resulting concentrated residue and distilled again under reduced pressure. Vacuum drying was then performed for 24 hours to obtain an ammonium salt additive 4 of Formula 4 below:

¹H-NMR (DMSO-d6): 7.48 (d, 2H), 7.12 (d, 2H), 5.05 (broad, 3H), 3.69 (m, 3H), 2.59 (m, 6H), 2.28 (s, 3H), 1.42 (m, 9H)

Example 1

A substrate on which a silicon oxide (SiOx) film deposited at a thickness of 500 angstroms (Å) and a silicon nitride (SiN) film deposited at a thickness of 5000 Å was prepared on a semiconductor wafer.

As shown in Table 1, 99.4 wt of phosphoric acid with 85% concentration, 0.5 wt % of 3-aminopropylsilanetriol, and 0.1 wt % of the ammonium salt additive 1 were added and mixed to prepare an etching composition such that a total weight percentage becomes 100 wt %.

Examples 2 to 4

The same method used in Example 1 was used to prepare an etching composition, except that ammonium salt additives 2 to 4 were used.

Comparative Example 1

The same method used in Example 1 was used to prepare an etching composition, except that no ammonium salt additive was used.

Comparative Example 2

The same method used in Example 1 was used to prepare an etching composition, except that ammonium acetate ((NH₄) OAc) was used as an ammonium salt additive.

<Preparation of Etching Composition and Measurement of Selection Ratio>

Each of the etching compositions obtained in Examples 1 to 4 and Comparative Example 1 was added to a round flask, heated for 60 minutes to heat up to 158° C. The silicon wafer was then dipped into the etching composition for 720 seconds and 6000 seconds to perform an etching process.

A surface of the silicon wafer, on which a pattern was formed, was selectively etched, and thicknesses of the silicon oxide film and the silicon nitride film before and after etching were measured using Ellipsometry, thin film thickness measurement equipment (NANO VIEW, SEMG-1000). Based thereon, an etching speed of the silicon oxide film (SiO E/R, Å/min) and that of the nitride film (SiN E/R, Å/min) as well as selection ratios thereof were calculated, and a result thereof is shown in Table 1.

The selection ratio represents a ratio of a nitride film etching speed to an oxide film etching speed, and the etching speed is a value calculated by dividing a difference in the film thicknesses before and after etching by an etching time (minute).

TABLE 1 Composition (wt %) Ammonium 85% salt Process Etching phosphoric 3-aminopropyl additive temp. SiN E/R SiO E/R selectivity acid silanetriol No./content (° C.) (Å/min) (Å/min) Ratio CE 1 99.5 0.5 wt % — 158 68.3 0.32 213 CE 2 99.4 0.5 wt % (NH₄)OAc/ 158 72.7 0.21 346 0.1 wt % EX 1 99.4 0.5 wt % 1/0.1 wt % 158 87.9 0.12 733 EX 2 99.4 0.5 wt % 2/0.1 wt % 158 87.3 0.11 794 EX 3 99.4 0.5 wt % 3/0.1 wt % 158 85.6 0.09 951 EX 4 99.4 0.5 wt % 4/0.1 wt % 158 91.3 0.15 609 *CE: Comparative Example **EX: Example

As shown in Table 1 above, it can be understood that remarkably high etching selectivity ratios are exhibited when ammonium salt additives 1 to 4 are added to the etching composition as an additive, as compared to Comparative Examples 1 and 2. Further, in terms of the silicon nitride etching speed (SiN E/R), the etching compositions of Examples 1 to 4 exhibited a remarkably excellent effect as compared to that of Comparative Examples 1 and 2. As a result, it is confirmed that an etching composition optimized for an etching process of a silicon nitride film can be provided.

Based on the results, it is confirmed that use of the ammonium salts suggested in the present disclosure as additives may lead to improved active silicon-based structural stability. Accordingly, an etching composition having an improved etching speed of a silicon nitride film, a selection ratio and etching stability can be provided.

[Change in Etching Composition Over Time]

The etching compositions of Comparative Example 1 and Example 1 were stored at about 70° C. for a predetermined period of time and subject to an etching test every 7 days under the same conditions. A result thereof is shown in Table 2.

TABLE 2 Period of SiN E/R SiO E/R Selection storage (Å/min) (Å/min) ratio Comparative — 68.3 0.32 213 Example 1  7 days 66.7 0.35 191 14 days 65.1 0.36 181 21 days 61.3 0.39 157 Example 1 — 87.9 0.12 733  7 days 87.2 0.12 727 14 days 86.6 0.12 722 21 days 86.5 0.12 721

As shown in Table 2 above, the etching composition of Comparative Example 1 after 21 days showed significantly reduced etching speeds (SiN E/R) and selection ratio. However, the etching composition of Example 1, to which the ammonium salt additive 1 is added, showed almost no change in the etching speeds (SiN E/R) and selection ratio. This indicates that the etching composition according to an embodiment of the present disclosure has not only excellent etching speeds and selection ratio but also excellent storage stability, and thus can maintain superior etching characteristics even when stored for a long period of time.

Such results also indicate that the structural stability effect of ammonium salt additive 1 can maintain the function of inhibiting etching of SiO without abnormal growth or decomposition of 3-aminopropylsilanetriol.

According to the aforementioned example embodiments, the etching composition according to the present disclosure has a high etching selectivity ratio of a nitride film to an oxide film.

In addition, use of the etching composition of the present disclosure may prevent damage of film quality of the oxide film when removing the nitride film or deterioration of electric characteristics due to etching of the oxide film, as well as preventing generation of particles, thereby improving device characteristics.

While exemplary embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present invention as defined by the appended claims. 

What is claimed is:
 1. An etching composition, comprising: phosphoric acid, a silane compound containing at least one silicon (Si) atom, and an ammonium salt represented by Formula 1 below:

wherein: L¹ to L³ are independently substituted or unsubstituted hydrocarbylene, R¹ to R⁴ are independently hydrogen, a substituted or unsubstituted hydrocarbyl group, and X^(n−) is an n-valent anion, where n is an integer of 1 to
 3. 2. The etching composition of claim 1, wherein X is one selected from the group consisting of a halogen, C₁-C₁₀ carboxylate, tosylate, sulfate, mesylate, bisulfate, carbonate, bicarbonate, phosphate, hydrogen phosphate, dihydrogen phosphate and nitrate.
 3. The etching composition of claim 1, wherein L¹ to L³ are independently substituted or unsubstituted C₁-C₅ alkylene.
 4. The etching composition of claim 3, wherein L¹ to L³ are the same.
 5. The etching composition of claim 4, wherein L¹ to L³ are —CH₂CH₂— or —CH₂CH(CH₃)—.
 6. The etching composition of claim 1, wherein the hydrocarbyl group is a substituted or unsubstituted C₁-C₂₀ alkyl group or a substituted or unsubstituted C₅-C₂₀ aryl group.
 7. The etching composition of claim 1, wherein R¹ to R³ are hydrogen, and R⁴ is hydrogen or a C₁-C₂₀ alkyl group.
 8. The etching composition of claim 7, wherein R⁴ is hydrogen or a —CH₃.
 9. The etching composition of claim 1, wherein Formula 1 is selected from following Structural Formulae 1 to 14:

where OAc, OTs and OMs represent acetate, tosylate and mesylate, respectively.
 10. The etching composition of claim 1, wherein the silane compound is represented by Formula 2 below:

wherein, in Formula 2, R⁵¹ to R⁵⁴ are independently hydrogen, a substituted or unsubstituted hydrocarbyl group or a substituted or unsubstituted heterohydrocarbyl group, and R⁵¹ to R⁵⁴ exist independently or two or more thereof form a ring via a heteroatom.
 11. The etching composition of claim 1, comprising 70 wt % to 95 wt % of the phosphoric acid, 0.001 wt % to 5 wt % of the silane compound and 0.001 wt % to 10 wt % of the ammonium salt represented by Formula
 1. 12. A method for etching an insulating film using the etching composition of claim
 1. 13. A method for preparing a semiconductor device, comprising: forming a tunnel oxide film, a polysilicon film, a buffer oxide film and a pad nitride film on a substrate in said order; forming a trench by selectively etching the polysilicon film, the buffer oxide film and the pad nitride film; forming an SOD oxide film to gap-fill the trench; performing a chemical mechanical polishing (CMP) process on the SOD oxide film using the pad nitride film as a polishing stop film; removing the pad nitride film by wet etching using the etching composition of claim 1; and removing the buffer oxide film. 